The invention relates generally to data communications, and more particularly, to a linear block interleaver for suppressing impulse noise in a discrete multi-tone (DMT) communications environment.
Data communication typically occurs as the transfer of information from one communication device to another. This is typically accomplished by the use of a modem located at each communication endpoint. In the past, the term modem denoted a piece of communication apparatus that performed a modulation and demodulation function, hence the term xe2x80x9cmodemxe2x80x9d. Today, the term modem is typically used to denote any piece of communication apparatus that enables the transfer of data and voice information from one location to another. For example, modern communication systems use many different technologies to perform the transfer of information from one location to another. Digital subscriber line (DSL) technology is one vehicle for such transfer of information. DSL technology uses the widely available subscriber loop, the copper wire pair that extends from a telephone company central office to a residential location, over which communication services, including the exchange of voice and data, may be provisioned. DSL devices can be referred to as modems, or, more accurately, transceivers, which connect the telephone company central office to the user, or remote location, typically referred to as the customer premises (CP). DSL communication devices use different formats and different types of modulation schemes and achieve widely varying communication rates. However, even the slowest DSL communications devices achieve data rates far in excess of conventional point-to-point modems.
Some of the available modulation schemes include quadrature-amplitude modulation (QAM), carrierless amplitude/phase (CAP) and DMT. In a DMT modulation scheme, a number of carriers, commonly referred to as xe2x80x9ctonesxe2x80x9d are encoded with the information to be transmitted and communicated over the communications channel. This information, in the form of data words, is encoded into signal space constellations and then transmitted. In a typical DMT transmitter, 256 carrier tones are used to encode the data and are added together resulting in a very high peak signal power due to the high numerical peak resulting from the addition of the 256 tones. Power consumption is further increased due to the use of square signal space constellations used in conventional DMT transmitters and the allowed +2.5 dB to xe2x88x9214.5 dB power variation allowed on each carrier tone. Square signal space constellations have an inherently high peak signal power due to the location of the highest power signal point. The peak signal power in conventional DMT transmitters is sufficiently high to cause saturation or clipping of the transmitter in normal operation. Conventional DMT allows a probability of clipping of approximately 10xe2x88x927. The number of bits encoded on each DMT carrier is selected in whole bit increments to bring the margin associated with each tone to within approximately 3 dB of the specified margin. Contributing to the peak power problem, the DMT modulation scheme allows the power on individual tones to be increased by up to approximately 2.5 dB to satisfy margin requirements. While increasing the power on some tones, the system reduces the power on other tones to maintain the specified transmit power. This scheme of tone power variation is useful in instances where it may be desirable to turn off specified tones and allocate their power to other tones. Unfortunately, this tone power variation results in spectrum management difficulties. Conventional DMT systems simply turn off specified tone carriers and increase others by the allowed 2.5 dB, but this results in the undesirable situation in which some carriers will be approximately 2.5 dB hotter than necessary in certain spectral bands, resulting in undesirable cross-talk, while other carrier tones are switched off completely. The tones at the high end of the frequency spectrum are frequently switched off.
Noise on individual DMT carrier tones and impulse noise cause major performance impairments to DSL modems. In systems that use DMT modulation, impulses are generated when the high peak power of the transmit signal saturates the digital-to-analog (DAC) in the transmitter, even prior to transmission. To combat this inherent deficiency, conventional DMT transmitters use expensive Reed-Solomon forward error correction encoders combined with bit-wise interleavers. Unfortunately, these coders introduce a significant amount of throughput delay.
Furthermore, noise is an ever present obstacle to optimal receiver performance. Noise imparted by the communication channel can be substantially eliminated through the use of well known techniques, such as precoding and channel equalization. Local noise imparted to a channel, such as periodic impulse noise, from local sources, such as electrical appliances and light dimmers, and random impulse noise, such as switching relays in a central office (CO), present an even greater problem that can degrade receiver performance.
All DSL equipment is susceptible to these impairments. In order to combat cross-talk, carrierless amplitude/phase (CAP) modulation uses a precoder, while discrete multi-tone (DMT) selectively disables the affected frequency bins, or tones. The 60 Hz periodic impulse noise (for example, that generated by a local electrical appliance such as a light dimmer) and other impulse noise generated for example by a switching relay at the central office is allowed to exist and the resulting errors are corrected by a Reed Solomon (RS) forward error correction code, sometimes in combination with a bit-wise interleaver. Unfortunately, as mentioned above, this solution adds throughput delay.
Other error correction codes are available which can be used to reduce some of the errors caused by impulse noise. For example, block codes, and more specifically, linear block codes have been developed for use in communications systems to correct or reduce the number of burst errors in data transmission. However, these codes have excess delay and have been unable to successfully reduce or eliminate the type of random impulse noise generated by relay switching equipment in a telephone company central office.
Therefore, it would be desirable to provide a noise suppression system and method in both a transmitter and a receiver of a DMT communication system to reduce or eliminate the impulse noise imparted to a receiver, without introducing throughput delay, and that eliminates the need for, but is compatible with, forward error correction.
The invention enables a communication device to efficiently suppress impulse noise. This noise can be for example, switching relay noise imparted to the communication channel at a telephone company central office or can be low frequency noise generated in the vicinity of a transceiver by, for example, a 60 Hz light dimmer.
The invention allows impulse noise to be eliminated from a transceiver employing DMT modulation by mathematically combining, or interleaving, a number of carrier tones and transmitting the combined energy of all the tones on each carrier tone. In this manner, impulse noise present on any one of the tones is spread among all the interleaved tones.